How It Works: Heat Recovery Ventilator

A simple device that keeps heat in while moving stale air out.

While necessity may be the mother of invention, it's increasing costs that spawn efficiency. Before the '70s, we happily cranked up the thermostat when the house felt chilly. Once heating costs went through the roof, though, we all put on sweaters and started looking for ways to save. And, with up to 40 percent of our heating dollar going to air infiltration—otherwise known as drafts—sealing the place up began to seem like the best defense against high heating bills.

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Over a period of time, older homes began to sport new, tight windows and doors, insulation and vapor-barrier improvements, modern siding, and caulk for every crack through which air might pass. New homes left the drawing board designed to be tight, and builders became familiar with the new materials and skills needed to meet market demand and updated regulations. Homes were finally becoming thermally efficient. What some began to wonder, though, was whether they were habitable.

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It turns out that those heat-robbing drafts had a role in the ecosystem of the home—they provided fresh air to breathe. Without realizing it, builders before the energy crisis had been installing an effective, albeit haphazard, ventilation system. If you could afford the heating bills, it worked.

Why Ventilate?

Life inside today's tight home generates both moisture and pollutants. The moisture comes from cooking, washing, showers and breathing. At excessive levels, moisture condenses on windows and can cause structural deterioration. Areas of excessive moisture are also breeding grounds for mold, mildew, fungi, dust mites and bacteria. You know you have a problem if you find moisture collecting on your windows, or if you notice black spots on walls. These unsightly spots indicate mildew growth. Mold spores and dust easily become airborne and circulate freely throughout the house, possibly causing a range of symptoms and allergic reactions.

In addition to excessive moisture and biological contaminants, appliances that utilize combustion have the potential for allowing gases, including carbon monoxide, and other pollutants to escape into the air. Some common sources may include gas ranges and water heaters, unvented space heaters, leaky chimneys and wood-burning appliances. Even breathing can add to the problem when carbon dioxide reaches excessive levels, creating stale air.

And that's not all that gets into the air. If your home is new, the very products it's made of can give off gases that are less than agreeable to your comfort and good health, and in many areas of the country there's a concern about radon seeping from the ground.

The American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) sets the standard for residential ventilation at a minimum of .35 air changes per hour, and not less than 15 cubic feet per minute (cfm) per person. An old home may very well exceed these values—especially on a windy day. However, on a calm winter day, even a drafty house may fall below the recommended minimum ventilation standard.

There are partial solutions to the indoor air-quality problem. For example, an electrostatic filter installed in a forced-air heating system will reduce airborne contaminants, but it won't help with moisture, stale air or gaseous pollutants. And, local exhaust fans can remove excess moisture in the kitchen, bath and laundry area, but create negative pressure inside the house. As they pump air out, the resultant vacuum slowly draws air into and through the house structure, bringing with it odors, dust and contaminants. In areas where radon is a problem, the negative pressure may increase radon levels.

A better whole-house solution is to create balanced ventilation. This way, one fan blows the stale, polluted air out of the house while another replaces it with fresh. Of course, if the fresh air is cold, you need to warm it up, and that costs money.

Holding The Heat

A heat-recovery ventilator (HRV) is similar to a balanced ventilation system, except it uses the heat in the outgoing stale air to warm up the fresh air. A typical unit features two fans—one to take out household air and the other to bring in fresh air. What makes an HRV unique is the heat-exchange core. The core transfers heat from the outgoing stream to the incoming stream in the same way that the radiator in your car transfers heat from the engine's coolant to the outside air. It's composed of a series of narrow alternating passages through which incoming and outgoing airstreams flow. As the streams move through, heat is transferred from the warm side of each passage to the cold, while the airstreams never mix.

Depending on the model, HRVs can recover up to 85 percent of the heat in the outgoing airstream, making these ventilators a lot easier on your budget than opening a few windows. And, an HRV contains filters that keep particulates such as pollen or dust from entering the house. You will, though, find your energy bill going up slightly to pay for replacing the heat that isn't recovered. An average HRV installation can run from $2000 to $2500, but costs will vary widely depending on the specific situation.

Although an HRV can be effective in the summer months, when it will take heat from incoming fresh air and transfer it to stale air-conditioned exhaust air, it's most popular in colder climates during the winter. If the temperature falls below about 20Â° F, however, frost can build up inside the exchange core. To handle this, a damper closes off the cold airstream and routes warm air through the core. After several minutes, a timer opens the fresh-air port and ventilation continues.

A typical HRV for residential use might move as much as 200 cfm of air, but the fan speed can be set to suit the air quality in the home. For example, a slow to medium fan speed may be adequate for normal living, while a house full of guests might require the highest setting. Controls are available for intermittent and remote operation.

HRVs are ideal for tight, moisture-prone homes because they replace the humid air with dry, fresh air. In climates with excessive outdoor humidity, an energy-recovery ventilator is more suitable. This device is similar to an HRV, but dehumidifies the incoming fresh airstream.